The world demand for inherently flame resistant synthetic fibres has increased sharply over the last few years. This trend is a result of increasing safety awareness of the population specially in highly developed and industrialized countries. Consequently, a number of special process technologies have been developed to produce combustion resistant textile products by finishing or dope additive techniques. In general the flame retardant chemicals are selected from the family of compounds of phosphorous, antimony, sulphur, halogens etc. However, many of these are not eco-friendly due to their hazardous nature.
Sandoz-Switzerland had developed a series of Pyrophosphate compounds for flame resistant additives. Out of these Sandoflam 5060 (2,2-oxybis(5,5-dimethyl-1,3,2-dioxaphosphorinane-2,2-disulphide) was found very suitable to produce flame retardant viscose fibre with a dose of 20% on cellulose. However the cost of this chemical is very high and thus the process becomes highly cost intensive. Moreover the product containing this compound on burning produces toxic gases.
On the other hand, the products containing silicon dioxide are considered out standing in their fire resistant characteristics. Such products are manufactured by number of methods. In some method, silica (SiO2) fibres are prepared by dry spinning method from a water glass solution as described in U.S. Pat. No. 4,786,017 and U.S. Pat. No. 4,332,601. These methods produce the fibres of Silicon-dioxide but do not contain the natural or synthetic polymers like cellulose.
In GB Patent 1064271, wherein sodium silicate is mixed with viscose (cellulose xanthate solution) and regenerated in to cellulose embedded with silicic acid. The fibres manufactured according to this method contained large amount of silica and modified with aluminium compounds like sodium aluminate. These fibres are further given a heat treatment in a muffle to convert it in to porous sintered fibres of SiO2. The cellulosic fibres containing silica, as produced by above method show serious disadvantages when given an alkaline treatment. In alkaline washing treatment the silica content dissolves out and the flame retarding effect gets nullified after some washings.
The method of prior arts for producing silica containing flame retardant fibre is zinc based process and the product contains zinc ions. In a typical commercial product the zinc content was found in the level of 50 ppm. Zinc is a known hazardous and toxic element and in many eco-label products it is restricted. The threshold limit of zinc in M.S.T. (Marke Scadstoffgeprufter Textilien) standard and HELCOM (The Helsinki Convention) for eco-labelling textiles is 5 and 2 ppm respectively. Many Environmental protection agencies like U.S. Environmental Protection Agency (EPA), The Occupational Safety and Health Administration (OSHA), Agency for Toxic Substance and Disease Registry (ATSDR) etc. have reported the ill effect of zinc compounds. Thus the end products made from the fibres/filaments containing zinc are not considered as eco-friendly and therefore unacceptable in some textile usage.
Moreover these products also show after glow behavior i.e. they glow spontaneously without flame.
Another major drawback of sodium aluminate treatment in the manufacturing of said product in above methods is the problem of precipitate and dust as some of the aluminates get precipitated in the form of alumina tri hydrate (Al2O3.3H2O) during the application stage which is insoluble in water or alkali. The content of these precipitates increases during the circulation of the solution and a major portion is carried over by the fibre mats. These precipitates adhere on the pressing rollers, thus making the movement of fibre mats difficult. The precipitates carried over by the fibre mats get dried in the dryers and spread as inorganic dust in nearby zone making the working atmosphere inconvenient and unfriendly. The dust problem is also observed in down stream processing of these products.
Therefore the product according to prior art methods suffers from the major drawbacks of having unacceptable levels of zinc, showing after glow behaviour, and generate inorganic dust.